The most westerly of the three valleys to which we have alluded is only
a slight depression of the surface of the land marked by a line of
_oases_. The depression is not sufficient to admit the waters of the
Mediterranean, nor are there any rains over any portion of the valley
which it forms sufficient to make it the bed of a stream. Springs issue,
however, here and there, in several places, from the ground, and,
percolating through the sands along the valley, give fertility to little
dells, long and narrow, which, by the contrast that they form with the
surrounding desolation, seem to the traveler to possess the verdure and
beauty of Paradise. There is a line of these oases extending along this
westerly depression, and some of them are of considerable extent. The
oasis of Siweh, on which stood the far-famed temple of Jupiter Ammon,
was many miles in extent, and was said to have contained in ancient
times a population of eight thousand souls. Thus, while the most
easterly of the three valleys which we have named was sunk so low as to
admit the ocean to flow freely into it, the most westerly was so
slightly depressed that it gained only a circumscribed and limited
fertility through the springs, which, in the lowest portions of it,
oozed from the ground. The third valley--the central one--remains now to
be described.

The reader will observe, by referring once more to the map, that south
of the great rainless region of which we are speaking, there lie groups
and ranges of mountains in Abyssinia, called the Mountains of the Moon.
These mountains are near the equator, and the relation which they
sustain to the surrounding seas, and to currents of wind which blow in
that quarter of the world, is such, that they bring down from the
atmosphere, especially in certain seasons of the year, vast and
continual torrents of rain. The water which thus falls drenches the
mountain sides and deluges the valleys. There is a great portion of it
which can not flow to the southward or eastward toward the sea, as the
whole country consists, in those directions, of continuous tracts of
elevated land. The rush of water thus turns to the northward, and,
pressing on across the desert through the great central valley which we
have referred to above, it finds an outlet, at last, in the
Mediterranean, at a point two thousand miles distant from the place
where the immense condenser drew it from the skies. The river thus
created is the Nile. It is formed, in a word, by the surplus waters of a
district inundated with rains, in their progress across a rainless
desert, seeking the sea.

[...]
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